WASHINGTON — When President Trump doled out A-pluses to his administration for hurricane response, declaring the government “totally prepared” to deal with a potentially devastating storm this week, it fell to a little-known adviser standing just to his left in the Oval Office to give a more somber assessment.

“Let’s set the expectations,” said Brock Long, the administrator of the Federal Emergency Management Agency, warning that Hurricane Florence could be “very devastating.”

Gesturing at a weather chart and speaking with a distinct southern drawl, he predicted weeks of power failures, flooding and displacement from homes, a somber warning about a storm that Mr. Trump had just referred to, almost admiringly, as a “tremendously large and tremendously wet” event.

As the storm has begun lashing the East Coast, Mr. Long, 43, a plain-spoken hurricane expert and emergency management veteran, has become the face of a high-stakes effort by Mr. Trump to cope with a potentially devastating natural disaster, and in the process, to repair the political damage from a much-criticized response to last year’s hurricane season that raised questions about his administration’s competence.

The assignment has a personal dimension for Mr. Long, who was born in North Carolina and recalls the day in 1989 when Hurricane Hugo swept over his home — he was 14 at the time — felling trees and buildings. “We were out of power for 10 days, I was out of school, I remember, for two weeks,” Mr. Long, wearing a navy-blue collared shirt emblazoned with the FEMA logo, told CBS on Tuesday. “This storm is setting up to be very similar to that one.”

But it is a particularly steep challenge for Mr. Long, whose agency has acknowledged shortcomings in its response last year to Hurricane Maria in Puerto Rico, where nearly 3,000 deaths were eventually attributed to the storm. Mr. Trump has fanned the flames of controversy over that episode as Hurricane Florence approaches, disputing the death toll and charging that Democrats had inflated the number for political reasons.

And on Thursday, just as Mr. Long had hoped to keep public attention focused on preparing for the potentially catastrophic storm, he had to confront a crisis of another kind: the revelation that the inspector general at the Department of Homeland Security, which includes FEMA, was looking into whether he had misused agency vehicles to commute from Washington to North Carolina, where his family resides.

News of the investigation forced Mr. Long to pivot, at least for a moment, to personal damage control, making an unscheduled appearance on a Thursday morning telephone briefing about the storm to acknowledge the investigation, which was reported by Politico.

“I would never intentionally run a program incorrectly,” Mr. Long said during the call, as the Category 1 storm was growing in size on its approach toward North Carolina. “Doing something unethical is not in my DNA.”

Mr. Long said he would “fully cooperate” with the investigation, “own up to any mistakes, and move on. But right now we’re going to get to Florence and move forward and concentrate on lifesaving issues.”

FEMA abruptly ended the briefing after reporters pressed Mr. Long about the investigation, and officials there referred questions to the inspector general’s office, which declined to comment.

“We are aware of the allegations and will review the I.G. report when it is complete,” Hogan Gidley, the deputy White House press secretary, said in a statement. “However, right now the administration is working nonstop to prepare a massive federal government support effort for those impacted by Hurricane Florence.”

The details of the investigation are far from clear. Former officials who insisted on anonymity to discuss security matters said the FEMA administrator must have access at all times to classified communications equipment, and in certain cases, that means sending a government car equipped with such capabilities with the agency’s chief when he travels. Mr. Long commutes to North Carolina on the weekends to be with his wife and two sons at his home in Hickory, not far from the house where he lived as a teenager when Hugo hit.

But colleagues and friends said Mr. Long was a principled person who was not known for cutting any corner, ethical or otherwise. They described an emergency management geek who was passionate about the intricacies of risk assessment and preparedness — he was once FEMA’s point man for contraflow traffic management, what happens when travel lanes are reversed during emergency evacuations — and likes to cut to the chase in crisis situations, peppering his all-hands emails with the phrase “bottom line.”

“Brock is the epitome of common sense,” said Thomas Bossert, Mr. Trump’s former homeland security adviser. “It’s that slow, Alabama drawl that he has, but he gets right down to the root of the matter and he’s a practical, solutions-driven kind of manager, so what you see is what you get. He’s an operator’s dream.”

According to his official biography, Mr. Long graduated from Appalachian State University and worked in Georgia’s emergency management agency and then in hurricane-related positions at FEMA before becoming Alabama’s top emergency management official in 2008. In 2011, he became a private contractor, and Mr. Trump nominated him last year to oversee FEMA.

Mr. Long “endeared himself” to Mr. Trump with his matter-of-fact briefings full of colorful visuals and frank assessments, and ready answers to the president’s often rapid-fire questions, Mr. Bossert said.

“They’re both the type of straight shooter in the private setting that will say it like it is, even if the language gets colorful,” Mr. Bossert said. “The president trusts him to be a competent manager, and Brock just doesn’t shy away from any challenge.”

But the job has also had its share of awkward moments. Last fall, at the height of the criticism of the federal response to Hurricane Maria, Mr. Long was in the Oval Office with Gov. Ricardo Roselló of Puerto Rico when Mr. Trump gave his government “a 10” for handling the storm and pointedly asked in front of reporters, “Did we do a great job?”

It fell to the FEMA chief to, in his words, “put this into context for America.” The agency had been working on disasters in 20 different states over 50 days registering almost four million people for individual assistance. “That’s more than Katrina, Rita, Wilma and Sandy combined,” Mr. Long said then. “It’s been a tremendous effort.”

Mr. Long was credited with directing successful efforts by FEMA in Texas and Florida, which despite some bureaucratic hiccups, have largely begun to recover and rebuild, unlike Puerto Rico and the Virgin Islands, which are still struggling a year after the storms.

A report that FEMA released in July detailed problems plaguing the response to Hurricane Maria, for which the agency has been heavily criticized, but Mr. Long praised its overall performance, and said his takeaway was that state and local governments had to do more to be ready for disastrous storms.

In a letter opening the report, Mr. Long wrote that he was “incredibly proud of how we performed in extraordinary circumstances” during the 2017 storm season. “While FEMA has and will continue to work with all levels of government to get much needed commodities to survivors, the hurricanes also showed that governments need to be better prepared with their own supplies, to have prepositioned contracts with enforcement mechanisms, and to be ready for the financial implications of a disaster.”

Mr. Long has said he is haunted by the notion that Americans have “hazard amnesia,” having forgotten what it is like to live through a major landfall hurricane like Katrina in 2005. “Sometimes I think we forget the worst,” he told a group of governors last year.

It was a message he repeated this year when he spoke to a group of students at Harvard studying crisis management.

“You can tell that he’s passionate about emergency management, and he cares about people,” said Rich Serino, the former deputy administrator of FEMA who teaches the course. “And he worries — we all do — that unless it’s affecting them directly, people can forget, and if they haven’t lived through storms, sometimes they don’t appreciate it.”

In an interview on Wednesday, before the internal investigation into Mr. Long’s conduct was revealed, Mr. Bossert made a point of praising him as an “ethical contractor,” who, as a private-sector consultant “had a reputation for being exceedingly fair, even to the point of probably underselling himself back to the government.”

Art Faulkner, who has known Mr. Long for about 10 years and succeeded him as director of Alabama emergency management agency, said he did not believe that the FEMA administrator would knowingly violate government rules.

“I know Brock Long, and if he in any way felt like what he was doing was illegal or unethical, he wouldn’t have done it,” said Mr. Faulkner, who is now a consultant in Elmore, Ala. “I can say that with 100 percent certainty.”

A version of this article appears in print on , on Page A21 of the New York edition with the headline: FEMA Chief’s Dual Challenge: Hurricane and an Investigation. Order Reprints | Today’s Paper | Subscribe